This invention relates to the production of fine stable latexes of block copolymers of vinyl aromatic hydrocarbons and conjugated dienes. More specifically, it relates to the production of such latexes which are stabilized with a non-migrating acrylic block polymer stabilizer.
It is known that a block copolymer can be obtained by an anionic copolymerization of a conjugated diene compound and an alkenyl arene compound by using an organic alkali metal initiator. Block copolymers have been produced which comprise blocks of these different monomers such as configurations which are linear, radial or star, i.e. many arms radiating from a central core. The proportion of thermoplastic blocks to elastomeric blocks and the relative molecular weights of each of these blocks is balanced to obtain a rubber having unique performance characteristics.
It has been found advantageous to prepare latexes of these polymers in order to obtain products that can be formulated into coatings and adhesives without the need for organic solvents. In some cases, small amounts of organic solvents are preferred to control evaporation of water or to serve as coalescing aids. But the quantity of organic solvent needed to control evaporation or aid coalescence will be much less than that needed to make corresponding coatings and adhesive films from organic solvent solutions (solvent-borne coatings and adhesives). Therefore, these latexes are comprised of the polymer, surfactants, coalescing solvents and water.
Preparation of such latexes is well known to those skilled in the art (see U.S. Pat. Nos. 3,360,599, 3,238,173 and 3,726,824). Latexes of polyvinyl aromatic-polydiene block copolymers are conventionally made by (1) dissolving the block copolymer in an organic solvent or solvents to form a polymer cement, (2) emulsifying the polymer cement in an aqueous solution containing one or more surfactants, and (3) stripping the emulsion of organic and excess aqueous liquids to form a stable, suitably concentrated latex. Polymer cements are often available directly from the polymerization reactor. Surfactants are usually present such that the total concentration of surfactants ranges from about 0.2 to more than 20 parts per hundred rubber (phr), where the rubber is the block copolymer. Such surfactants may be low molecular weight conventional surfactants including anionic, cationic, and nonionic surfactants or they may be polymeric surfactants having one or more water soluble blocks. Low levels of organic solvents are often left in the latex and/or other solvents are added following latex concentration as aids to control evaporation of water or aid coalescence and film formation after application. Therefore, conventional latexes of block copolymers are usually comprised of polymer, surfactants, organic solvents, and water.
Surfactants are capable of migrating through the polymer matrix during film formation since they are not irreversibly anchored in any one location. Therefore, high surfactant concentration can negatively impact adhesive properties of adhesives and lead to moisture ingress in coatings. Those interested in water-based emulsion adhesives and coatings have attempted to solve this problem by using less surfactant but some surfactant must be used in order to form the emulsion. Thus, it would be highly advantageous to be able to make a block copolymer emulsion and latex with a non-migrating stabilizer.
In some cases, fine polymer cement emulsions are necessary in order to make fine latexes. The present invention can produce fine emulsions using a non-migrating polymeric stabilizer. It is also known that small (submicron) particle sizes in latexes can enhance coating and adhesive film coalescence. Thus, it is highly advantageous to be able to produce a stable polymer cement emulsion with an average drop size of less than one micron.